There are known in the prior art electrophotographic copying machines in which a drum carrying a surface layer of photoconductive material is driven to carry the photoconductive material successively past a charging station at which a uniform electrostatic charge is placed on the surface, then through an exposure station at which the charged photoconductor is subjected to a radiant image of an original to be copied selectively to discharge the photoconductive layer to produce a latent electrostatic image and then through a developing station at which the latent image is subjected to the action of a developer. The developer may either be a dry developer or a liquid developer. In each case, following development of the image there is directed onto the surface of the drum in synchronism with the leading edge of the developed image a sheet of copy material to which the image is to be transferred. The developed image and the sheet of copy material are carried by the drum past a transfer device, such for example as a corona, which causes the developed image to migrate from the surface of the drum to the copy sheet.
After the developed image has been transferred to the copy sheet, the sheet must be removed from the surface of the drum and delivered to the machine user. This operation, particularly where liquid developers are used, is one of the most difficult operations to achieve with a high degree of reliability. Various arrangements have been proposed in the prior art for removing the copy sheet carrying the developed image from the drum.
One of the more reliable arrangements for removing the copy sheet carrying the developed image from the drum and for delivering it to the machine user is shown and described in Ariyama U.S. Pat. No. 3,936,045. In the arrangement shown in that patent, a sheet of copy material to which the image is to be transferred is fed to the drum at a location at which its leading edge is in registry with the leading edge of the image on the drum. The sheet moves together with the image under a transfer corona. In the sheet stripping or pickoff arrangement disclosed in this patent, a thin, narrow metal strip extends over a portion of the periphery of the drum at the rear of the machine in the region of the transfer corona. As the copy sheet is fed to the drum, about seven millimeters of the width of the sheet from the rear edge rides on the thin metal strip rather than on the drum. It will readily be appreciated that, owing to the interposition of the metal strip between the under surface of the copy sheet and the drum at the rear edge, any portion of the developed image which might exist on the drum under the metal strip cannot be transferred to the copy sheet. Thus, there results a narrow band of copy "deletion" along the edge of the copy sheet at the rear of the machine.
After emerging from beneath the corona, the portion of the copy sheet riding along the metal strip engages a struck-up portion thereof which directs the leading rear corner of the sheet up into the nip between a belt and a turn roller which carry the sheet around and reverse its direction to deliver it to the operator.
It will readily be appreciated that owing to the thickness of the metal strip, the portion of the sheet immediately forward of the front edge of the strip is held away from the drum so that development in this region is relatively uncertain and a greater deletion of image than is necessary to achieve pickoff may result. The patentee, in order to obviate this result suggests that a wire carried by the same rear frame plate bracket as that which supports the pickoff blade be biased toward the drum at a location just forward of the forward edge of the pickoff blade so as to hold the copy paper against the drum in this region.
While the pickoff system just described is relatively reliable in removing the copy from the drum, it incorporates a number of disadvantages.
Owing to the fact that the pressure wire contacts the drum it tends to collect toner particles. This condition is aggravated by toner which is picked up by the wire in the course of the reverse development cleaning cycle or anti-deposition cycle which the development electrode of the machine goes through between each copy. This condition further is aggravated when relatively thick toners are used in the developing process.
Toner which is collected on the pressure wire in the manner described hereinabove is deposited on the back of a copy as the copy sheet runs under the pressure finger in the course of an image transfer operation. This produces an unsightly streak down the back of the copy which is particularly objectionable where copying is being done on both sides of the sheet.
The condition described above worsens with time since the pressure wire casing wears down and develops a flat providing a greater surface area around which the liquid developer leaves a meniscus producing an even larger mark on the back of a copy. This condition requires the pressure wire to be cleaned on each service call. Owing to the fact that the pressure wire normally is mounted on the rear frame plate of the machine it is relatively inaccessible and difficult to service.
In addition to the foregoing problem, the photoreceptor may be damaged as a result of the pressure wire riding directly on the surface thereof when no sheet of copy paper is present. A further danger exists of corona electrical arcing as a result of a breakdown of the insulating cover on the pressure wire.